This invention relates to a process for producing microporous polypropylene membranes and to the membrane so-produced.
Microporous membranes include thin sheets and hollow fibers generally formed from synthetic thermoplastic materials and having a substantially continuous matrix structure containing open pores or conduits of small size. The size range for pores of "microporous membranes" is not precisely defined in the art, but it is generally understood to extend from about 0.05 microns to about 10 microns.
It is disclosed in U.S. Pat. Nos. 4,247,498 and 4,519,909 as well as in European Patent Application No. 83306651.7 and 83306652.5 that polypropylene microporous membranes can be formed by casting under conditions to effect a thermally induced phase separation. A mixture of polypropylene and an initially incompatible liquid is heated to form a homogeneous solution which is then cooled under non equilibrium thermodynamic conditions to initiate liquid-liquid phase separation. The polypropylene is solidified by further cooling and the liquid dispersed within the solid matrix is removed by extraction to form a microporous structure. It is disclosed that different microporous structures can be formed by varying the cooling rate. A slow cooling rate produces a cellular microstructure whereas a rapid cooling rate results in a lacey noncellular structure. It is disclosed that the choice of solvents is critical and only those which will result in liquid-liquid phase separation are useful. The solvents which exhibit solid (crystalline)-liquid separation behavior are not useful in producing microporous structure. In particular, dioctyl phthalate is specifically disclosed as being a nonuseful solvent in these processes. U.S. Pat. No. 4,539,256 and 4,519,909 also discloses a method for forming microporous membranes from polypropylene at column 13, lines 17-28. In the process, between 30 and 80 weight percent of a polymer is mixed with a solvent at an elevated temperature high enough to dissolve the polypropylene. Thereafter the mixture is cooled to effect crystallization of the polypropylene and a shaped article such as a film is formed during the cooling. The article is subsequently subjected to stretching (orientation) in order to produce the desired product. The resultant product is microporous characterized by a multiplicity of spaced, randomly dispersed non-uniform shaped, nonporous particles of the polypropylene. Adjacent particles throughout the material are separated from one another to provide micropores and which particles are connected to each other by a plurality of fibrils consisting of the polypropylene. Suitable solvents include mineral oil and dioctyl phthalate. Since this product must be oriented or highly stretched during its formation, it cannot be deposited directly on a substrate which cannot be highly stretched such as a woven, nonwoven or knitted substrate.
It would be desirable to provide a process for producing microporous polypropylene membranes which permit the use of solvents which effect a solid (crystalline)-liquid phase separation since it would promote solubilization of the polymer in the solvent. In addition, it would be desirable to provide such a process which produces a product having uniform porosity and which eliminates the need for stretching of the product during formation so that the product can be deposited directly upon a suitable substrate such as a porous woven or unwoven substrate which cannot be stretched. If such a process were available, the membrane could be formed onto the porous substrate directly in order to produce laminates having the increased mechanical strength which would permit their use in a wide variety of filtration environments.